API St 520-1-2020 (2023) PDF

Full title and description

API Std 520, Part 1 — Sizing, Selection, and Installation of Pressure‑relieving Devices — Part I: Sizing and Selection. Tenth edition (published 1 October 2020) with Errata 1 (May 2023). This standard provides rules, methodologies and worked examples for sizing and selecting pressure‑relief devices (PRDs) used to protect unfired pressure vessels and related equipment in refineries and similar process facilities.

Abstract

API Std 520‑1 establishes the technical principles and calculation procedures used to determine required relieving capacity and to select appropriate pressure‑relief devices for equipment having a maximum allowable working pressure (MAWP) of 15 psig (103 kPag) or greater. The standard covers definitions, operating characteristics, steady‑state sizing methods for Newtonian fluids, and guidance to document and justify PRD selection. It does not cover atmospheric/low‑pressure storage tanks (see API 2000) or overpressure protection of fired vessels governed by ASME Section I and other codes.

General information

• Status: Active (current edition with published errata).
• Publication date: 1 October 2020 (Tenth edition); Errata 1 published May 2023.
• Publisher: American Petroleum Institute (API).
• ICS / categories: 13.240 (Protection against excessive pressure); 23.020 (Fluid storage devices / pressure vessels); 23.060 (Valves).
• Edition / version: 10th edition (API Std 520, Part 1) — includes Errata 1 (May 2023).
• Number of pages: 172 (standard document length for the 10th edition).

Scope

This standard applies to the sizing and selection of pressure‑relief devices used in refineries and related industries for equipment with MAWP ≥ 15 psig (103 kPag). It addresses relief scenarios from normal operating upsets and fire/emergency heating, presents calculation methods based on steady‑state flow for Newtonian fluids, defines required relief loads and accumulation allowances, and specifies documentation and selection criteria for PRDs. It excludes atmospheric/low‑pressure storage tanks and fired‑vessel overpressure rules covered by other codes.

Key topics and requirements

• Definitions and classification of pressure‑relieving devices (spring‑loaded valves, pilot‑operated valves, rupture disks, balanced valves, etc.).
• Relief load determination for common overpressure scenarios (operational, process upsets, blocked lines, thermal expansion, fire exposure) and accumulation allowances.
• Sizing procedures and equations for steady‑state discharge of Newtonian fluids (vapor, liquid, two‑phase considerations and limitations).
• Selection criteria for PRD type, set pressure, built‑in blowdown, and allowable backpressure considerations.
• Requirements for documentation, tagging, and records to demonstrate correct sizing/selection and traceability.
• Notes on interaction with related codes (ASME pressure vessel and piping codes) and limits of applicability (what is excluded or covered by other API standards).

Typical use and users

Used by process, mechanical and safety engineers, relief‑system designers, plant operators, inspection authorities and PRD manufacturers. Typical applications include refinery process vessels, separators, reactors, tanks (where within scope), pressure‑containing equipment in chemical plants and any facility requiring documented overpressure protection design as recognized good engineering practice.

Related standards

API Std 520, Part 2 (Installation) — complements Part 1 by covering installation and piping of PRDs; API Std 521 — Guide for Pressure‑relieving and Depressuring Systems; API Std 526 — Flanged Steel Pressure‑relief Valves; API Std 527 — Seat Tightness of Pressure Relief Valves; ASME BPVC (Section VIII) and ASME B31 piping codes; ISO/IEC and EN standards on safety valves and bursting discs (for international equivalence).

Keywords

pressure‑relief device, PRD, safety valve, relief valve, rupture disk, sizing, selection, overpressure, accumulation, API 520, API 521, ASME, refinery safety

FAQ

Q: What is this standard?

A: API Std 520‑1 (Part 1) is the industry standard that defines how to size and select pressure‑relief devices for unfired pressure vessels and related equipment in refineries and similar process plants. The current (10th) edition was published 1 October 2020 and includes Errata 1 issued May 2023.

Q: What does it cover?

A: It covers definitions, operational characteristics, relief load determination, steady‑state sizing methods for Newtonian fluids, selection criteria for PRD type and settings, and documentation requirements. It does not replace installation requirements (see Part 2) nor does it cover atmospheric tanks or fired vessels governed by other codes.

Q: Who typically uses it?

A: Process and mechanical engineers, relief system designers, plant operations and maintenance staff, equipment manufacturers, and regulatory/inspection personnel use this standard to ensure proper overpressure protection and to document that relief systems meet recognized good engineering practice.

Q: Is it current or superseded?

A: The 10th edition (published 1 October 2020) is the current edition; an official Errata 1 was released in May 2023. Users should apply the 2020 text together with the May 2023 errata unless a newer revision is published.

Q: Is it part of a series?

A: Yes. API 520 is a multi‑part standard: Part 1 covers sizing and selection; Part 2 covers installation. API 521 (guide to pressure‑relieving and depressuring systems) and other API documents (526, 527, etc.) form a related set addressing different aspects of overpressure protection.

Q: What are the key keywords?

A: Pressure‑relief device, PRD, relief valve, rupture disk, sizing, set pressure, accumulation, overpressure protection, API 520, API 521.